Method of fabricating anisometric metal needles and birefringent suspension thereof in dielectric fluid

ABSTRACT

A grating (22) of narrow parallel ridges (24) is formed in the surface (26) of a substrate (20) made of a hard material such as silicon dioxide. Metal (40) is deposited onto the grating (22) perpendicular to the ridges (24) at an angle of approximately 45° to the surface (26) of the grating such that the metal (40) is deposited onto the top (24a) and one of the sides (24b) of the each of the ridges (24) to form generally L-shaped metal strips (12&#39;) thereon. The metal strips (12&#39;) are cut at periodic intervals along the ridges (24) to produce anisometric metal needles (12). The substrate (20) is immersed in a dielectric fluid (14), and ultrasonic energy is applied to cause the needles (12) to release from the substrate (20) into suspension in the fluid (14). The L-shape of the needles (12) makes them resistant to bending. The suspension ( 10) has birefringent properties similar to liquid crystals, but may be electrically switched at much higher speed. The index of refraction of the suspension (10) varies in accordance with the alignment of the needles (12), thereby enabling the direction and phase of an electromagnetic wave propagating through the suspension (10) to be controlled by varying the magnitude of the applied electric field.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to electronic and electro-opticdevices, and more particularly to the fabrication of anisometric metalneedles for a birefringent suspension in a dielectric fluid.

2. Description of the Related Art

Devices for beam steering, beam focusing, waveguiding, and phaseshifting are essential components of radar and communications systems.

Ferrites and diode arrays are generally used as phase shifters inmicrowave radar systems, as described in a textbook entitled "RADARHANDBOOK, 2nd Edition", by M. Skolnik, ed., Mc-Graw Hill Publishing Co.,1990, pp. 7.63-7.67. These devices work well for narrow, tuned spectralbands, but not for the wide bands desired for frequency-agile systems.The phase shift of ferrites is sensitive to operating temperatures. Thehigh power consumption of diodes prohibits their use for high powermicrowave antennas. These existing types of phase shifters are alsoexpensive, which is a major concern in an application such as a phasedarray radar system in which thousands of phase shifters may be required.

Beam steering devices for the infrared region generally include liquidcrystals as birefringent elements, but these are relatively slow foradvanced electro-optic applications.

SUMMARY OF THE INVENTION

The present invention provides a method of producing a birefringentsuspension of anisometric metal needles in a dielectric fluid, which canbe used as a controlling element in beam steering, beam focusing,waveguiding, and phase shifting devices for electronic and opticalapplications. Where the length of the needles is very small relative tothe wavelength of the electromagnetic waves being controlled, theneedles act as dipoles having very large dipole moment. This enables theorientation of the needles in the fluid to be shifted from a randomstate in the absence of an electric field to an aligned state uponapplication of an electric field in a very short period of time, on theorder of milliseconds.

The index of refraction of the suspension varies in accordance with thealignment of the needles, thereby enabling the phase of anelectromagnetic wave propagating through the suspension to be controlledby varying the magnitude of the applied electric field.

The present suspension has birefringent properties similar to liquidcrystals, but may be electrically switched at much higher speed. TheL-shape of the needles makes them resistant to bending. The suspensionis especially suited for use as a lens or waveguide for the infrared tomicrowave region of the electromagnetic spectrum, exhibiting a flatresponse curve over wide spectral regions. Microwave applicationsinclude near object detection (NOD), true ground speed sensing, andother radar systems for automotive vehicles.

In accordance with the present method of fabricating anisometric needlesand a suspension thereof in a dielectric fluid, a grating of narrowparallel ridges is formed in the surface of a substrate made of a hardmaterial such as silicon dioxide. Metal is deposited onto the grating atan angle of approximately 45° to the surface of the grating andperpendicular to the ridges of the grating such that the metal isdeposited onto the top and one of the sides of each of the ridges toform generally L-shaped metal strips thereon. The metal strips are cutat periodic intervals along the ridges to produce the needles. To form asuspension directly, the substrate is immersed in a dielectric fluid,and ultrasonic energy is applied to cause the needles to release fromthe substrate into suspension in the fluid. To form a mass of needles,the substrate is immersed in a solvent and the needles caused to bereleased from the substrate into suspension in the solvent. The solventis then evaporated or otherwise removed to leave the mass of needles,which may be stored and transported in dry form, and subsequently mixedinto a dielectric fluid for use.

These and other features and advantages of the present invention will beapparent to those skilled in the art from the following detaileddescription, taken together with the accompanying drawings, in whichlike reference numerals refer to like parts.

DESCRIPTION OF THE DRAWINGS

FIGS. 1a to 1c are diagrams illustrating how the index of refraction ofa birefringent suspension of anisometric metal needles fabricated inaccordance with the present method can be varied by means of an appliedelectric field;

FIG. 2 is a perspective view of an anisometric metal needle fabricatedin accordance with the present method;

FIG. 3 is a simplified plan view of a substrate formed with a grating ofparallel ridges for practicing the present method;

FIGS. 4 to 8 are simplified fragmentary sectional views illustratingsteps of the present method;

FIGS. 9 and 10 are simplified fragmentary plan views illustratingfurther steps of the present method;

FIG. 11 is a simplified fragmentary sectional view illustrating a finalstep of the present method; and

FIG. 12 is a plan view illustrating a mass of separate needlesfabricated in accordance with the present method.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, metal needles are suspended ina dielectric fluid to achieve high birefringence with fast response timeand low voltage and low drive power activation. The needles createoptical birefringence in the fluid in a manner similar to that of thedirectors in a liquid crystal material. When the length L of a metalneedle is more than an order of magnitude smaller than the wavelength ofan incident electromagnetic wave, the metal needle exhibits induceddielectric dipole moment characteristics.

The phenomenon which makes the needle act as a dipole and not as aconductive wire is that the current induced in the needle in response toan applied electrical potential E is proportional to dE/dt. A constantvalue of E generates only transient current, which produces a positiveor negative electric charge Q at the opposite ends of the needle. Thecharge Q generates a field, which when superimposed on the externalfield, produces a zero internal field. Conversely, in a wire there is aconstant current for a constant field. The phase of the current in theneedle is shifted by 90° relative to the field, enabling the suspensionto produce optical phase modulation. In a wire, the current is in phasewith the electric field.

The needles in the present suspension are designed to have an aspectratio (length to width ratio) in excess of five and preferably ten ormore. The dipole moment has a value of QL, which is the same as for amolecular dielectric dipole moment. In the case of the needle, thecharge Q is proportional to the applied potential E, and the length L isconstant. Conversely, for a permanent dielectric dipole moment, Q isconstant.

There is also a large difference in the magnitude of the dipole moment.In a typical dielectric dipole, L is of molecular size (10-100Angstroms), whereas in the case of the present needles, L is muchlarger, on the order of 5,000-40,000 Angstroms. The value of Q for adielectric dipole is usually equal to one molecular charge, or oneelectron. In the case of a needle, the value of Q may be on the order of10¹⁵ electrons. The dipole moment associated with a metal needle istherefore very large compared to molecular dipole moments.

A birefringent suspension according to the present invention may be usedin several different modes of operation, an exemplary one of which isillustrated in FIGS. 1a to 1c. The birefringent suspension fabricated inaccordance with the present invention is designated as 10, and includesa plurality of anisometric metal needles 12 suspended in a dielectricfluid 14 which is collectively designated by circles in the drawings. Anelectric field may be applied across the suspension 10 by means ofelectrodes 16 and 18. For a low value V0 (such as zero volts) ofelectrical potential applied to the electrodes 16 and 18, the needles 12will be randomly aligned in the fluid 14 as in FIG. 1a, and thesuspension 10 will have a relatively high index of refraction.

In response to a large value of V2 of applied electrical potential, theneedles 12 will be aligned with the electric field, as shown in FIG. 1c.Assume that the direction of propagation of the electromagnetic wave isperpendicular to the plane of the paper in the drawing. The suspension10 will have a relatively low index of refraction if the plane ofpolarization is perpendicular to the long axis of the needles, and arelatively high refractive index if the plane of polarization isparallel to the long axis of the needles. For an intermediate value V1of applied electrical potential (FIG. 1b), the needles will be partiallyaligned and the index of refraction will have an intermediate valuebetween those produced by the potentials V0 and V2.

An anisometric metal needle 12 fabricated in accordance with the presentinvention is illustrated in FIG. 2. The needle 12 has a generallyL-shaped cross section. However, the needles 12 need not necessarilyhave sharp angled sides, but may have a curved, crescent shape. Thecross-section of the needles 12 has an inwardly facing, concave surface12a, and an outwardly facing, convex surface 12b.

Although the needle 12 may be constituted by a single piece of metal, itis preferably formed of two layers to facilitate fabrication using thepresent method. The needle 12 as illustrated in FIG. 2 includes a firstmetal layer 12c, and a second metal layer 12d formed on the outwardlyfacing convex surface of the first metal layer 12c. In a preferredembodiment of the needle 12, the first metal layer 12c is formed ofcopper, and the second metal layer 12d is formed of aluminum.

Although the needles 12 can be made from any good electrical conductor,aluminum is the most preferred material. It is an excellent conductor,and its density can be matched to suitable dielectric fluids. Thedensity of the needles 12 need not necessarily be matched exactly to thedensity of the liquid, but will preferably be close to it. Also, thesurface of aluminum is spontaneously oxidized in air to a depth of about50 Angstroms, although this can be increased to about 150 Angstroms byheating if desired. The oxidation is self-limiting in that it does notcontinue readily beyond this point.

Another applicable metal is magnesium. It is chemically less stable insuspension, but lighter than aluminum, and thereby less difficult tomatch to the density of a dielectric fluid.

Aluminum is not a ferromagnetic material. For applications in which aferromagnetic material is desired, the preferred materials are iron ornickel. Iron and nickel have densities greater than that of allavailable dielectric fluids, but the average density of the needles canbe reduced by coating them with a low-density polymer.

The length L of the needle 12 is preferably at least five and morepreferably ten times the width W and height H thereof, with W and Hbeing approximately equal, as illustrated in FIG. 2. The length L isbetween approximately 5,000-40,000 Angstroms, and the width W and heightH are between approximately 500-10,000 Angstroms. The thickness of theneedles 12, equal to the combined thicknesses of the layers 12c and 12d,is between approximately 200-3,000 Angstroms. Where copper and aluminumare used for the layers 12c and 12d respectively, the thickness of thecopper layer 12c is between approximately 10-100 Angstroms and thethickness of the aluminum layer 12d is between approximately 100-3000Angstroms.

The needles 12 constitute between approximately 0.01%-10% of thesuspension by volume. The dielectric fluid 14 may include any stable,organic dielectric liquid, including ketones, alcohols, esters,aliphatic and aromatic hydrocarbons, aliphatic and aromatic halocarbons,aldehydes, amines, and combinations thereof. However, for applicationsin which the birefringence of the suspension 10 is important, the fluid14 should have low absorption of the electromagnetic radiation that isbeing controlled.

For the millimeter wave and microwave regions of the electromagneticspectrum, it can be important for the fluid 14 to have substantially nopermanent dipole, since molecules with permanent dipoles absorb some ofthe radiation in these regions. Molecules that have no permanent dipoleinclude aliphatic hydrocarbons, aliphatic perfluorocarbons, andsymmetrically substituted organic molecular materials.

For these applications, specific preferred liquids are aliphatichydrocarbons including pentane, hexane, heptane, octane, nonane, decane,undecane, dodecane, tridecane, tetradecane, pentadecane, hexadecane, andisomers thereof. Other preferred liquids are cycloaliphatic hydrocarbonsincluding cyclohexane and decalin.

Further preferred liquids are symmetrical aromatic hydrocarbonsincluding benzene, para-xylene, mesitylene, and 1,3,5-triethylbenzene.Carbon tetrachloride is also a suitable liquid.

To stabilize the suspension of the needles 12 in the fluid 14, it isdesirable to use liquids with a density as close as possible to that ofthe metal of the needles. For this reason, it may be desirable toutilize fluids with high density. This may be accomplished by includingone or more solid organic dielectric materials in one or a combinationof the preferred dielectric liquids. Preferred solid organic materialsfor this purpose which have a symmetrical chemical structure and nopermanent dipole include carbon tetrabromide, para-dichlorobenzene,1,3,5-trichlorobenzene, 1,2,4,5-tetrachlorobenzene,1,3,5-tribromobenzene, and 1,2,4,5-tetrabromobenzene.

For applications in the infrared region of the electromagnetic spectrum,it is usually not important for the liquid to consist of molecules withno permanent dipole moment. It is only important for the liquid to havelow absorption in the operating wavelength range. For such applications,very dense liquids, such as bromoform, 1,1,2,2-tetrabromoethane, and1,1,1,2-tetrabromoethane may be suitable.

The choice of metal or metals for the needles 12, and the liquid orcombination of liquids for the fluid 14, depends on the particularapplication, and on the stability of the needles 12 in the fluid 14. Thepresent invention should not be construed as limited to the particularmetals or dielectric liquids listed above.

A method of fabricating a mass of the needles 12, and the suspension 10including the needles 12 suspended in the dielectric fluid 14, will nowbe described with references to FIGS. 3 to 10 of the drawings.

FIG. 3 illustrates a substrate 20 formed with a grating 22 of parallelridges 24 formed on a surface 26 of the substrate 20. The surface 26 isconsidered to be the base plane of the upper peripheral boundary of thesubstrate 20 as viewed in the drawings, and is defined by differentmaterial layer structures as the processing proceeds. Preferably, thegrating 22 is formed in such a manner that the substrate 20 may becleaned and reused for practicing the present method many times. Only afew ridges 24 are shown as being formed on the surface 26 of thesubstrate 20 in FIG. 3 for clarity of illustration. However, it will beunderstood that in actual practice of the present method, a much largernumber of ridges will be formed thereon.

The substrate 20 may be formed entirely of an oxide of silicon, such assilicon dioxide, or silicon nitride, or include a silicon wafer with thesurface 26 subjected to a suitable oxidization process. Althoughmaterials other than silicon oxides may be used, it is necessary thatthe grating 22 be sufficiently rigid and durable to practice the presentmethod at least once.

The grating 22 may be formed in the surface 26 of the substrate 20 asillustrated in FIGS. 4 to 6. As shown, the substrate 20 includes asilicon wafer 21, and a layer 23 of silicon dioxide approximately 5,000Angstroms thick formed on the wafer 21. In FIG. 4, a layer ofphotoresist 28 is deposited on the surface 26, and exposed to an opticalpattern 30 which corresponds to the grating 22. The pattern 30 may begenerated holographically using a laser hologram generator 32, or anyother optical means.

As illustrated in FIG. 5, the photoresist 28 is developed to form aphotoresist pattern 34 consisting of ridges 36 which were not removed bythe developer solution. Typically, the period of the pattern 30,corresponding to the lateral spacing between the centers of adjacentridges 24, may be on the order of 5,000 Angstroms, with the widths ofthe ridges 24 being approximately equal to the spaces between the ridges24. For smaller needles, gratings with smaller periods must be used.

In FIG. 6, an etchant is applied to etch the surface 26 of the substrate20 in the areas of the photoresist pattern 34 which were removed by thedeveloper, and which are thereby not covered by the ridges 36. Theetching forms depressions which define the ridges 24 therebetween. Theetching will typically be performed so that the ridges 24 will have aheight of approximately 5,000 Angstroms. Where the silicon dioxide layer23 is 5,000 Angstroms thick, the etching will cut through the oxidelayer 23 down to the underlying silicon, such that the upper surface ofthe silicon wafer 21, which is not affected by the etchant, now definesthe surface 26. The photoresist pattern 34 is then stripped away toproduce the substrate 20 as illustrated in FIG. 3. A number ofphotoresists, developers, and etchants suitable for performing the stepsof FIGS. 5 and 6 are commercially available, and the choice thereof isnot critical to the practice of the invention.

As illustrated in FIG. 7, metal 40 is deposited onto the grating 22 toform generally L-shaped metal strips 12' which extend perpendicular tothe plane of the drawing and will be cut as described below to producethe metal needles 12. The metal 40 is preferably deposited at an angleof about 90° to the ridges 24, and an angle of approximately 45° to thesurface 26 as illustrated by arrows. A preferred deposition process iselectron beam evaporation, although the invention is not so limited. Theridges 24 have tops 24a, first sides 24b, and second sides 24c. Themetal 40 is deposited to form the first layer 12c and second layer 12ddescribed above and illustrated in FIG. 2. The first layer 12c is firstevaporated onto the tops 24a and upper portions of the first sides 24bof the ridges 24. The first layer 12c is preferably copper, and isprovided to facilitate removal of the needles 12 from the surface 26after fabrication. Where removal is not a problem, the first layer 12cmay be omitted. The second layer 12d, preferably of aluminum, is thenevaporated onto the first layer 12c.

The first layer 12c may be deposited to a preferred thickness of 50Angstroms, but the thickness thereof may have a value betweenapproximately 10-100 Angstroms within the scope of the invention. Thesecond layer 12d may be deposited to a preferred thickness of betweenapproximately 750-1000 Angstroms, but the thickness thereof may have avalue between approximately 100-3000 Angstroms within the scope of theinvention.

The angle of deposition relative to the surface 26 is approximately 45°,but may vary within a range in which the metal 40 is deposited to asufficient extent onto the first sides 24b of the ridges 24, but not inthe spaces between adjacent ridges 24. Where the period and depth of thegrating 22 are both approximately 5,000 Angstroms, the width W andheight H of the metal strips 12' will be approximately 2,500 Angstroms,although these values may vary from between approximately 500-10,000Angstroms within the scope of the invention.

FIGS. 8, 9 and 10 illustrate the steps for cutting the metal strips 12'at periodic intervals I along their lengths to produce the metal needles12. In FIG. 8, a photoresist layer 42 is formed on the surface 26 of thesubstrate 20 to cover the metal strips 12', and is exposed to an opticalpattern of parallel lines as indicated at 44. The lines 44 extendperpendicular to the ridges 24, and are spaced from each other by theperiodic intervals I which correspond to the desired length L of theneedles 12. As shown in FIG. 9, each interval I is equal to L plus thewidth of each line 44.

In FIG. 9, the photoresist 42 is developed to form a photoresist pattern46 including areas 48 corresponding to the lines 44 in which thephotoresist 42 is removed by the developer solution. The metal strips12' and surface 26 of the substrate 20 are thereby exposed in the areas48. In FIG. 10, an etchant is applied to the surface 26 which etchesthrough the layers 12c and 12d of the metal strips 12' in the exposedareas 48 and thereby cuts the strips 12' into the desired length L. Theetchant does not, however, affect the silicon dioxide ridges 24 of thegrating 22, or the underlying bulk of the silicon substrate 20. Thephotoresist 42 is then stripped away to leave the resulting needles 12adhered to the tops 24a and first sides 24b of the ridges 24. The lengthL of the needles 12 is preferably between approximately 10,000-30,000Angstroms, but may vary from between approximately 5,000-40,000Angstroms within the scope of the invention.

A number of photoresists, developers, and etchants suitable forperforming the steps of FIGS. 8 to 10 are commercially available, andthe choice thereof is not critical to the practice of the invention. Theetchant used in the step of FIG. 10 may be a commercial aluminum etchingsolution including an acid or mixture of acids. Alternative etchingmethods include plasma etching and reactive ion etching.

As illustrated in FIG. 11, the substrate 20 is immersed in a solution 50which may be a solvent such as acetone, isopropyl alcohol, carbontetrachloride, etc. to release the needles 12 from the substrate 20.Ultrasonic energy 51 may be applied to assist in removal of the needles12 from the substrate 20. The solvent may be removed from the resultingmixture using evaporation, centrifugal force, or the like to produce amass 52 of separate needles 12 as illustrated in FIG. 12. The mass 52may be stored and transported in dry form, and subsequently mixed intoany of the dielectric fluids 14 described above to produce the presentbirefringent suspension 10.

Alternatively, the solution 50 itself may constitute the dielectricfluid 14, in which case the suspension 10 is produced as illustrated inFIG. 11 without the intermediate step of removing the solution 50 fromthe needles 12. Where the solution 50 (fluid 14) is a solvent such asacetone, the stability of the solution 10 may be enhanced by a residualcoating of photoresist 42 which causes the needles 12 to resistflocculation and settling in the fluid 14. Where the needles 12 aresuspended in other solvents, it may be desirable to coat them withanother polymer material to achieve a similar effect. However, theneedles 12 may be stably suspended in a number of the organic dielectricliquids listed above without the necessity of a polymer coating. In anycase, the substrate 20 is preferably cleaned for reuse after the needles12 are removed therefrom.

EXAMPLE

A grating was formed in the surface of a commercial silicon wafer whichwas 7.6 centimeters in diameter and had a 5,000 Angstrom thick layer ofsilicon dioxide formed thereon. The grating had a period of 5,000Angstroms, and a depth of 5,000 Angstroms. A first layer of copper and asecond layer of aluminum were deposited on the ridges of the grating atan angle of 45° using electron beam evaporation. The pressure in theevaporation chamber was 10⁻⁶ to 10⁻⁷ mmHg, the temperature of thealuminum source was 600° C. to 700° C., and the deposition rate wasapproximately 10 Angstroms per second. The copper layer was deposited toa thickness of approximately 100 Angstroms, and the aluminum layer wasdeposited to a thickness of approximately 750 Angstroms.

The surface of the substrate was coated with AZ-1350B photoresistmanufactured by the American Hoechst Company. The photoresist wasexposed by contact printing through a mask to an optical pattern oflines extending perpendicular to the ridges of the grating and having aperiod of approximately 40,000 Angstroms, and developed to expose theunderlying metal strips in the areas corresponding to the lines of theoptical pattern. A conventional aluminum etchant was used to dissolvethe metal strips in the exposed areas. The photoresist was removed usingacetone, with minimum agitation so that the metal needles remainedundisturbed on the ridges of the grating. The substrate was thenimmersed in a solution of clean acetone, and ultrasonic energy wasapplied to release the needles from the grating into solution in theacetone.

Approximately 2.3×10⁹ needles were produced, which had a length ofapproximately 20,000 Angstroms, width of approximately 2,500 Angstroms,and thickness of approximately 850 Angstroms. The needles remained instable suspension in the acetone solution without settling for over tenweeks.

While several illustrative embodiments of the invention have been shownand described, numerous variations and alternate embodiments will occurto those skilled in the art, without departing from the spirit and scopeof the invention. Accordingly, it is intended that the present inventionnot be limited solely to the specifically described illustrativeembodiments. Various modifications are contemplated and can be madewithout departing from the spirit and scope of the invention as definedby the appended claims.

We claim:
 1. A method of fabricating anisometric needles, comprising thesteps of:(a) providing a substrate having a grating of parallel ridgesformed in a surface thereof, each ridge having a top and first andsecond opposite sides; (b) depositing metal onto the grating at an angleselected such that said metal is deposited onto the tops and first sidesof the ridges to form generally L-shaped metal strips; (c) cutting themetal strips at periodic intervals along the ridges to form saidneedles; and (d) removing said needles from the grating.
 2. A method asin claim 1, in which step (a) comprises the substeps of:(e) providingthe substrate as having said surface initially flat; (f) forming a layerof photoresist on said surface; (g) exposing the photoresist with anoptical pattern corresponding to the grating; (h) developing the exposedphotoresist to form a photoresist pattern corresponding to the grating;(i) etching said surface in areas of the photoresist pattern which wereremoved by development in step (h) to form the grating; and (j) removingthe photoresist pattern from the grating.
 3. A method as in claim 2, inwhich step (g) comprises generating the optical pattern holographically.4. A method as in claim 2, in which step (e) comprises providing thesubstrate having the surface thereof formed of an oxide of silicon.
 5. Amethod as in claim 1, in which said angle in step (b) is approximately90° relative to the ridges and approximately 45° relative to the surfaceof the substrate.
 6. A method as in claim 1, in which step (b) comprisesdepositing said metal onto the grating using electron beam evaporation.7. A method as in claim 1, in which step (b) comprises depositing saidmetal onto the grating by depositing a first metal layer onto the topsand first sides of the ridges, and depositing a second metal layer ontothe first metal layer.
 8. A method as in claim 7, in which step (b)comprises depositing the first metal layer as including copper, anddepositing the second metal layer as including aluminum.
 9. A method asin claim 8, in which step (b) comprises depositing the first metal layerto a thickness of between approximately 10-100 Angstroms, and depositingthe second metal layer to a thickness of between approximately 100-3,000Angstroms.
 10. A method as in claim 1, in which step (c) comprises thesubsteps of:(e) forming a layer of photoresist over the metal strips andexposed areas of the grating; (f) exposing the photoresist with anoptical pattern of parallel lines which extend perpendicular to theridges of the grating and are spaced from each other by said periodicintervals; (g) developing the exposed photoresist to form a photoresistpattern corresponding to said parallel lines; (h) etching through themetal strips in areas of the photoresist pattern which were removed bydevelopment in step (g) to thereby cut the metal strips and form saidneedles; and (i) removing the photoresist pattern from the metal stripsand exposed areas of the grating.
 11. A method as in claim 1, in which(d) comprises immersing the substrate in a solvent and applyingultrasonic energy to the solvent and substrate.
 12. A method ofproducing a suspension of anisometric needles in a dielectric fluid,comprising the steps of:(a) providing a substrate having a grating ofparallel ridges formed in a surface thereof, each ridge having a top andfirst and second opposite sides; (b) depositing metal onto the gratingat an angle selected such that said metal is deposited onto the tops andfirst sides of the ridges to form generally L-shaped metal strips; (c)cutting the metal strips at periodic intervals along the ridges to formsaid needles; (d) immersing the substrate in said fluid; and (e) causingsaid needles to release from the grating into suspension in said fluid.13. A method as in claim 12, in which (e) comprises applying ultrasonicenergy to said fluid and the substrate.
 14. A method as in claim 12, inwhich step (d) comprises providing said fluid as including a materialselected from the group consisting of ketones, alcohols, esters,aliphatic and aromatic hydrocarbons, aliphatic and aromatic halocarbons,aldehydes, and amines.
 15. A method as in claim 12, in which step (d)comprises providing said fluid as including an organic material havingsubstantially no permanent dipole.
 16. A method as in claim 15, in whichstep (b) comprises selecting said organic material from the groupconsisting of aliphatic hydrocarbons, aliphatic perfluorocarbons, andsymmetrically substituted organic molecular materials.